The present invention relates generally to base station receivers for communication applications and specifically to a receiver front end for base stations used in mobile radio systems.
In terrestrial mobile radio systems of cellular, PCS or other type, geographical areas are subdivided into a number of cells. The communications traffic in each cell is supported by a base station and each base station has assigned to it a multiplicity of RF carriers. In such cellular mobile radio systems that operate at UHF and higher frequencies, the size of the cells is determined by terrain features (there can not be manor obstructions between the mobile station and the base station), network capacity requirements (the number of users the system needs to support), and the base station receiver sensitivity (limited by losses and noise generated in the base station receiver front end). In such cellular mobile radio systems, cells are called capacity cells when their size is determined by traffic requirements, and cells are called coverage cells when their size is determined by the base station receiver sensitivity and the terrain. Furthermore, a distinction is made between the forward link, which is the radio signal transmitted from the base station to the mobile station, and the reverse link, which is the radio signal transmitted from the mobile station to the base station.
In the reverse link, the mobile station typically transmits 10 to 100 times less power than the base station transmits in the forward link. Therefore,the received signal strength at the base station is much lower than the received signal strength at the mobile station. In situations where the base station range is limited by the reverse link signal strength, the base station is identified as reverse link limited. Likewise, in forward link limited cells the range is limited by the strength of the signal received at the mobile station.
Mobile radio networks are designed for balanced forward and reverse links, i.e., equal base station range in both directions. This balance is based on the assumption that all cells are at full capacity. However, many operational networks are not at full capacity. Under these conditions, the base station transmitter can be driven harder to provide an increased range for the forward link. The cell is then reverse link limited.
Specialized Mobile Radio (SMR) base stations and rural cellular base stations are typically reverse link limited. In particular, many existing cellular base stations are reverse link limited because they were designed for car phones transmitting at about 8Watts, while the majority of mobile stations today are battery operated hand-held phones, which transmit at much lower power levels (0.6 Watt in the U.S. and 2 Watts in Europe).
Reverse link limitations in specific existing cells due to terrain can be overcome. by increasing the antenna tower height at the base station. More general, construction of additional base stations or repeater sites is necessary. Both these approaches have major disadvantages: increasing the height of the receive antennas on the tower is typically not possible without replacing the entire tower and may violate zoning regulations. Building additional base stations or repeater sites is expensive and also requires a reassignment of the frequency reuse pattern of the network.
In capacity limited cellular networks, additional demand in the number of users can be met by adding new frequency channels to the existing cell sites if the additional channels are available. In networks where all channels are in use the only solution is splitting existing cells into smaller ones, and correspondingly, adding additional base stations and reassigning the frequency reuse pattern.
It is an objective of the present invention to disclose receiver front end circuitry that can provide significantly increased base station sensitivity for receiving reverse link signals from mobile stations. A related objective is to minimize the noise contributions from cable losses in the base station receive path which also increases the base station reverse link sensitivity compared to existing base stations.
Another objective is to reduce the number of base stations in coverage networks thereby reducing the installation and maintenance cost of such networks relative to existing cellular mobile radio systems.
Another related objective is to reduce the mobile station transmit power in coverage or capacity networks by increasing the base station receiver sensitivity.
It is a further objective to provide base station receiver front end circuitry with improved RF filter characteristics to reduce interference. This feature increases spectrum utilization providing increased capacity and revenue relative to existing base stations.
Yet another related objective is to operate said receiver front end circuitry in a thermally stable environment to avoid variations, degradation in performance, and failure due to ambient temperature fluctuations.
An additional objective relating to some digital cellular mobile radio systems is to increase network capacity. These and other objectives are achieved in the present invention which provides a receiver front end for a base station. The receiver front end includes: (1) a plurality of filtering means for spectrally filtering a plurality of RF signals to form a plurality of filtered RF signals; (2) a plurality of amplifying means, in communication with the plurality of filtering means, for amplifying the plurality of filtered RF signals; and (3) cooling means for cryogenically cooling the filtering means and the amplifying means. The cooling means is common to the plurality of filtering means and plurality of amplifying means and is substantially adjacent to the antenna to maintain the insertion loss along a transmission line extending between the antenna and amplifying means at or below a selected level. At least one of the plurality of filtering means and plurality of amplifying means comprises a superconducting material. In one embodiment, the receiver front end is mounted on a structure supporting the antenna. The cooling means can be a closed or open cycle refrigerator. The cooling means can maintain the filtering means and amplifying means at a stable temperature that is independent of the temperature of the environment external to the cooling means. The filtering means, amplifying means, and cooling means will hereinafter be referred to as the cryoelectronic receiver front end or the receiver front end. In one embodiment, the cryoelectronic receiver front end consists at a minimum of a spectral filter and a low noise amplifier, either or both of which can include a superconducting material for the passive components of the circuit.
To understand the performance advantages of the present invention, it is important to relate base station sensitivity with the base station noise figure. The sensitivity is described as the RF signal power level needed at the receive antenna port to detect a single telephone channel with a given signal quality. Frequently, in digital mobile radio systems, this signal quality is described by a frame error rate not exceeding one percent.
This error rate is a strong function of the signal to noise ratio as measured, for example, before the demodulator, and is thus strongly dependent on the noise power. The noise power in turn, is composed of noise received by the antenna and noise added by the RF receiver front end circuitry. The latter can be measured with standard techniques and is typically expressed as a noise figure value. The more noise added by the receiver, the larger the base station noise figure, the larger the total noise power at the demodulator, and the lower the sensitivity of the base station.
Cryogenic cooling significantly decreases RF losses in passive electronic circuits thereby reducing the thermal noise, also known as Johnson noise. As is also well known, Johnson noise generated in passive components is equal to the component loss when the component is operated at room temperature, but decreases substantially below the loss value when the component is operated at cryogenic temperature. Additionally, the losses in normal metals decrease with temperature, and the RF losses of superconducting metals when cooled below the transition temperature, are orders of magnitude lower than that of normal metals. The noise mechanisms intrinsic to a variety of semiconductor transistor designs, such as those used in low noise amplifiers, are also temperature dependent, and decrease with decreasing temperature. For example, the noise figure of PHEMT GaAs low noise amplifiers is known to substantially decrease when operated at cryogenic temperature. Preferably, the insertion loss of the filters is no more than about 0.2 dB and the noise figure of the LNAs is no more than about 0.4 dB at the temperature of the components (i.e., no more than about 150xc2x0 K) in the cryocooler.
In addition to the use of cryoelectronic components with extremely low noise temperature, in the present invention, the RF feed line losses between the receive antenna and the cryoelectronic receiver front end are substantially minimized by locating the cryoelectronic receiver front end on the antenna mast in close proximity to the receive antenna structure. In cellular base stations it is common practice to locate all base station electronics including the receiver front end at the base of the antenna mast. Depending on the height of the mast, a substantial length of RF feed line (typically coaxial cable) is used to connect the receiver front end to the antenna port. This cable causes insertion losses that directly add to the base station noise figure. In the typical embodiment of the invention, the cryoelectronic receiver front end is mounted on the antenna support structure itself. Preferably, the insertion loss along the transmission line extending between the antenna and the receiver front end is no more than about 1.0 dB and more preferably no more than about 0.5 dB.
The noise figure of the cryoelectronic receiver front end of the present invention is preferably no more than 1.5 dB, more preferentially no more than 1.0 dB, and most preferably no more than about 0.7 dB. This compares with noise figures in the range of 3 to 8 dB in existing base stations. With respect to base stations sensitivity, this corresponds to a 2 to 7 dB improvement over the existing state of the art. The concomitant increase in reverse link range in cellular applications is preferably at least about 110%, more preferably at least about 120%, and most preferably at least about 140% of the reverse link range of conventional systems (i.e., with no tower-mounted cryoelectronic receiver front end).
The use of superconducting material in the RF spectral filter provides not only high sensitivity but also improved spectral definition of the cellular band. Ideal bandpass filters have rectangular profiles. Actual filters have sloping skirts and in-band ripple. The low losses of superconducting material allow the fabrication of very small filter circuits with steep skirts and low in-band ripple. When used in mobile cellular radio system, such filters allow better use of the available spectrum, as more channels can be accommodated at the band edges without increased interference from adjacent bands. The small size of the superconducting planar filters allows use of more complex filter functions to be performed without increasing the size of the mast head cryoelectronic receiver front end and without significant loss in sensitivity. For example, combinations of bandpass and bandreject filters may be used in base stations where strong out-of-band interference signals need to be suppressed. Also, sharper filters can be used to more accurately define specific receive bands or parts thereof. For example, it is customary in the new cellular PCs systems to use 60-MHz wide filters. This corresponds to the entire PCS base station receive band. In actuality, each licensee only uses a small part of this spectrum, i.e., either a 15 MHz or a 5 MHz wide band. Superconducting filters can easily provide the selectivity for these narrower bands with only a minor increase in noise figure.
Another benefit of the cryoelectronic receiver front end is the increased spurious free dynamic range compared with existing receiver front ends. This is the result of the increase in amplifier gain and the decrease in noise realized through cooling the circuit.
The present invention is applicable to all base station modulation and multiplexing formats, such as analog or digital modulation, frequency, phase or amplitude modulation, frequency-, time- and code-division multiplexing. The improved sensitivity may be utilized in different cellular mobile systems in different ways. The benefits include but are not limited to: balancing of reverse link limited cells; increasing base station range in coverage networks; increasing cell capacity; better reception of signals transmitted through buildings and other structures; substantial reduction of degradation in receiver sensitivity caused by the insertion loss of the transmission line extending from the receiver front end to the base station because the RF signal is spectrally filtered and amplified before transmission along the line; and reducing handset transmit power levels for safety reasons, for increased talk time, and for better signal quality due to the higher linearity of the transmit amplifier.
The cryoelectronic receiver front end can readily be applied in cellular mobile radio systems designed to have balanced links. As the transmit power in the mobile stations is continuously adjusted to the minimum value for maintaining a certain reverse link quality, the use of the present invention allows mobile stations to operate at substantially reduced power levels. This increases the talk time for a given battery size, and reduces the power levels that users are exposed to.
In mobile radio systems that implement spread spectrum technology, such as code division multiplex systems, increased sensitivity of the base station receiver front end as provided with the present invention is known to significantly increase not only the cell size but also the capacity.
The filtering means and amplifying means in the receiver front end can be electronically tunable and/or located on a common substrate. The tuning means for tuning the filtering means can include a ferroelectric material.
The cooling means can include a cooling device, means for compressing a cooling fluid for use in the cooling device, and means for transporting the cooling fluid between the compressing means and the cooling device. The compressing means can be located near the base of the structure supporting the antenna, and the cooling device and receiver front end mounted on the upper part of the structure. In this manner, the compressing means can supply a number of cooling devices with cooling fluid. The cooling fluid can be transported to one or more cooling devices mounted on the structure by transmitting the cooling fluid through a conduit formed by the transmission line extending from the base station to the receiver front end.
The cooling means can include: (i) a cold finger contacting the plurality of filtering means and the plurality of amplifying means; (ii) valve means in communication with a conduit for providing the cooling fluid to the cold finger; (iii) variable speed motor means connected to the valve means for actuating the valve means at a variable frequency; (iv) a temperature sensor for sensing the temperature of the cold finger and providing an output signal representative of the temperature; and (v) means for controlling the speed of the variable motor means in response to the output signal of the temperature sensor. In this manner, the temperature of the cold finger is controlled by varying the speed of the variable motor means.
In another embodiment of the present invention, the cooling means includes a cooling member having a plurality of faces with at least one of the filtering means and at least one of the amplifying means being positioned adjacent to each of the number of faces. Typically, the cooling member has at least three faces. At least two filtering means and at least two amplifying means are typically adjacent to each of the faces. The cooling member is mounted on the cold finger at a location yielding the desired temperature of the cooling member.
In yet another embodiment of the present invention, the cooling means further includes a mounting means for mounting the amplifying means on the cooling member. The mounting means has a bulk conductivity sufficient to cause the amplifying means to have a higher temperature than the cooling member. Preferably, the frame member has a bulk conductivity of at least about 2 watts/cm-xc2x0 K, and an integrated thermal conductivity of 20 watts/cm or less.
To protect the receiver front end from the environment, it can be enclosed in a weatherproof enclosure. The input and output ports in the enclosure for electrical conductors can be protected from power surges, such as by lightning, by lightning protection means. The enclosure can form an integral structure with the antenna, particularly with a patch array antenna.
Switching means can be used to bypass the RF signal around the receiver front end in the event of malfunction of receiver front end. Monitoring means for monitoring remotely the operation of the various components of the receiver front end can be used to activate the switching means.
For dual diversity reception, a second cooling means in a second receiver front end can be employed. The antenna is in communication with the receiver front end and the second antenna with the second receiver front end. This configuration provides enhanced system reliability by providing separate cooling means for the receiver front ends in each sector.
In another configuration for servicing multiple antennas, a single cryostat can include a plurality of filtering means and a plurality of amplifying means. In this configuration, a filtering means and amplifying means are connected with each of a plurality of antennas.
The relative locations of the filters and amplifiers on the cold finger can be important for optimal performance of each component. In one embodiment, the amplifier is positioned nearer the free end of the cold finger than the filter. This configuration provides significantly reduced insertion loss in the coaxial cable between the antenna and the filter. In another embodiment, a temperature gradient exists along the length of the cold finger with the lowest temperature being at the free end of the cold finger. Because the filter has a lower optimum operating temperature than the amplifier, the filter is located nearer the free end of the cold finger than the amplifier. In either case, the filter or amplifier is located at the point on the temperature gradient with the desired optimum operating temperature.
In another embodiment, the present invention provides a method for processing a wireless signal transmitted by a mobile station to a base station. The method includes the steps of: (1) cryogenically cooling components of the base station""s receiver front end, with the temperature of cooling preferably being 90% or less of the transition temperature of a superconducting material in the receiver front end; (2) receiving the signal with the receiver front end; and (3) transmitting the received signal to the base station.